A method of using an image display device for electrically controlling the position of a mechanical shutter to provide image display (hereinafter, referred to as a “movable shutter-type image display device”) is described in US 2008/0174532.
FIG. 16 is a circuit diagram showing a pixel circuit of a conventional movable shutter-type image display device.
Hereinafter, the conventional movable shutter-type image display device will be described with reference to FIG. 16.
Each pixel circuit 213 includes a signal line 206. The signal line 206 and a signal storage capacitance 204 are connected to each other by a signal transfer switch 205.
The signal storage capacitance 204 is also connected to a gate of an n-type MOS transistor 203 for shutter negative voltage write. A drain of the n-type MOS transistor 203 for shutter negative voltage write is connected to a drain of a p-type MOS transistor 202 for shutter positive voltage write via a cascode n-type MOS transistor 216 and a cascode p-type MOS transistor 215.
Each pixel includes a dual actuator shutter assembly 201 connected to a shutter voltage line 211. One of two control electrodes of the dual actuator shutter assembly 201 is connected to the drain of the n-type MOS transistor 203 for shutter negative voltage write via the cascode n-type MOS transistor 216. The other control electrode is connected to a control electrode voltage line 209.
The other end of the signal storage capacitance 204 is connected to the shutter voltage line 211. A source of the n-type MOS transistor 203 for shutter negative voltage write is connected to an nMOS source voltage line 212 for shutter negative voltage write.
A gate and the drain of the p-type MOS transistor 202 for shutter positive voltage write are respectively connected to a pMOS gate voltage line 207 for shutter positive voltage write and a positive voltage line 208. A gate of the cascode n-type MOS transistor 216 and a gate of the cascode p-type MOS transistor 215 are connected to a cascode gate voltage line 217. A gate of the signal transfer switch 205 is connected to a scanning line 210.
The dual actuator shutter assembly 201 is provided to face an opening in a light blocking surface. This image display device includes a plurality of such pixel circuits 213 arrayed in a matrix.
Now, an operation of the conventional movable shutter-type image display device will be described.
An image signal voltage written to the signal line 206 is stored on the signal storage capacitance 204 via the signal transfer switch 205 by scanning the scanning lines 210 sequentially.
Then, after the scanning operation for writing an image signal voltage on the signal storage capacitances 204 of all the pixels is finished, each pixel performs amplified write of an image signal to one of the control electrodes of the dual actuator shutter assembly 201, based on the written image signal voltage. Namely, in all the pixels, the pMOS gate voltage line 207 for shutter positive voltage write is put into a low voltage state for a prescribed time period, so that the p-type MOS transistor 202 for shutter positive voltage write is put into an ON state for this period. Thus, one of the control electrodes of the dual actuator shutter assembly 201 is precharged with a prescribed positive voltage, which has been applied to the positive voltage line 208.
Next, the nMOS source voltage line 212 for shutter negative voltage write is put into a low voltage state for a prescribed time period. For this period, only in a pixel which includes the signal storage capacitance 204 having a high voltage as an image signal voltage written therein, the n-type MOS transistor 203 for shutter negative voltage write is put into an ON state. As a result, the voltage of one of the control electrodes of the dual actuator shutter assembly 201 is re-rewritten with a prescribed low voltage applied to the nMOS source voltage line 212 for shutter negative voltage write.
In a pixel which includes the signal storage capacitance 204 having a low voltage as an image signal voltage written therein, the n-type MOS transistor 203 for shutter negative voltage write is kept in an OFF state for the above-mentioned period. Therefore, the voltage of one of the control electrodes of the dual actuator shutter assembly 201 is kept in the state of being precharged at the prescribed positive voltage.
In this manner, amplified write of an image signal is performed on one of the control electrodes of the dual actuator shutter assembly 201. In parallel with this, a voltage applied to the control electrode voltage line 209 is controlled, so that the dual actuator shutter assembly 201 can be electrostatically operated to be opened or closed. The opening provided in the light blocking surface of the dual actuator shutter assembly 201 is opened or closed in this manner, so that the amount of transmitted light is controlled. Thus, the image display device can display an image corresponding to the written image signal voltage on the pixel matrix.
In the above-described operation, the cascode n-type MOS transistor 216 and the cascode p-type MOS transistor 215 are provided in order to protect the p-type MOS transistor 202 for shutter positive voltage write and the n-type MOS transistor 203 for shutter negative voltage write against write of a high drain voltage which may spoil the reliability life thereof.
It was found that in a movable shutter-type image display device, a control fault of a mechanical shutter which reduces the life of the device is caused by an adhesive force caused between the shutter electrode and the control electrode.
This will be described with reference to FIG. 6. FIG. 6 provides schematic views of a shutter electrode 20 and a control electrode 21 provided in each of pixels in a movable shutter-type image display device. An insulating film 50 of alumina or silicon nitride is provided around the electrodes.
FIG. 6(a) shows that the shutter electrode 20 is electrostatically attracted to the control electrode 21. A voltage of, for example, 25V is applied between the electrodes. At this point, a prescribed electric field is generated in the insulating film 50 located between the electrodes, and thus a leak current is generated due to Poole-Frankel or Fowler-Nordheim injection current.
Which of the current injection mechanisms mainly acts is determined by the quality of the film, electric field, temperature or the like. For example, in the case where the shutter electrode 20 is supplied with a negative voltage and the control electrode 21 is supplied with a positive voltage, the generated leak current is defined as electron injection directed from the shutter electrode 20 toward the control electrode 21. It should be noted that the insulating film 50 located between the electrodes has a contact interface and a great number of electron trap levels are existent around the contact interface.
Electrons released from the insulating film 50 on the shutter electrode 20 side are concentrated in tiny convexed portions on the insulating film, and therefore the influence of the electron trap is small. By contrast, on the control electrode 21 side, injected electrons are dispersed in a large area of the interface of the insulating film 50, and therefore a great number of electrons are trapped at the electron trap levels. This is shown in FIG. 6(b).
FIG. 6(c) shows a state where no voltage is applied between the electrodes after the state of FIG. 6(b). Even when there is no voltage applied anymore, electrons, once trapped, stay at the interface of the insulating film 50 for a relatively long period. Even when a negative electrode is kept applied to the shutter electrode 20 and a positive electrode is kept applied to the control electrode 21 in order to close the electrodes, the residual charges decrease the potential of the positive electrode. As a result, the control on the shutter performed by use of electrostatic attraction is destabilized. As a result, the life of the mechanical shutter or the life of the display device may be reduced.
The present invention for solving these problems of the conventional art has an object of providing a technology capable of, in a movable shutter-type image display device, significantly decreasing the amount of trapped electrons (charges) caused by a leak current to alleviate the control fault of the mechanical shutter and thus to significantly extend the life of the display.
The foregoing and the other objects and novel features of the present invention will be made clear by the description in this specification and the attached drawings.